The rheology of wormlike micelles ("worms") formed by surfactants in water often follows nonmonotonic trends as functions of composition. For example, a study by Raghavan et al. (Langmuir 2002, 18, 3797) on mixtures of the anionic surfactant sodium oleate (NaOA) and the cationic surfactant octyl trimethylammonium bromide (OTAB) reported a pronounced peak in the zero-shear viscosity η0 as a function of NaOA/OTAB ratio at a constant surfactant concentration (3 wt%). In this work, we study the origins of rheological changes in the NaOA/OTAB system and the relations between the composition and structural characteristics using cryo-transmission electron microscopy (cryo-TEM). When either surfactant is in large excess, the dominating morphology is that of spherical micelles. As oppositely charged surfactant is added to the mixture, the spheres grow into linear worms and these continue to elongate as the viscosity peak (which occurs at a 70/30 NaOA/OTAB ratio) is approached from either end. At the viscosity peak, the sample shows numerous long worms as well as a small number of branched worms. Taken together, NaOA/OTAB rheology can be primarily understood on the basis of micellar growth, which is explained primarily by packing arguments. While the size of the hydrophobic micellar core continuously decreases as the short amphiphile OTAB is added at the expense of NaOA, screening of charges goes through a maximum, which contributes to the asymmetry of the viscosity curve. With regard to micellar branching, there is no significant difference in the density of branched worms on either side of the viscosity peak. Therefore, it appears that in contrast to the behavior of some surfactant/salt systems, branching does not have a significant influence on the rheology of this mixed catanionic surfactant system. Instead, our data clearly indicate that the origin of the viscosity peak is linked with micellar growth and micellar shortening.